1. Field of the Invention
The present invention relates to a range image producing method, and apparatus for image taking. More particularly, the present invention relates to a range image producing method, and apparatus for image taking in which precision in measuring a distance can be high by suitably compensating for blank portions in a range image.
2. Description Related to the Prior Art
The time-of-flight method (TOF method) is known as optical measurement of a distance to an object. In the TOF, inspection light is applied to the object. The time of flight of the light is measured, namely the time from the emission until the light reception from the object. A distance to the object is determined according to the time of the flight.
A range image sensor and an apparatus for image taking having the range image sensor are known in the art of the rangefinding. The range image sensor forms a range image in which a distance to the object is expressed by a pixel value. For example, U.S. Pat. No. 6,057,909 (corresponding to JP-A 11-508359) discloses the range image sensor in which infrared pulsed light is applied to the object, and reflected infrared light from the object is received by an image pickup device. The range image is acquired according to time from the application to the reception and pixel values of the image signal. Also, U.S. Pat. No. 6,856,355 (corresponding to JP-A 2001-215110) discloses a type of the range image sensor in which infrared light is modulated by sine wave modulation, and is applied to the object. Reflected infrared light from the object is received by an image pickup device by 1/n shift of the phase, so as to acquire the range image.
Also, a chroma key method is known, in which the object in a field of view is extracted by separation from the background. In the chroma key method, color of the background is kept the blue color, so only the object of the foreground is extracted according to chroma components of the image. However, there is a shortcoming in that the preparation of the background color requires much time and work. The color of the object on the foreground is limited in relation with the blue color. It is conceivable that the range image sensor described above is used to extract an image in a distance range being predetermined. The use of the range image sensor makes it possible to extract an image in the distance range by use of a sequence and time of light reception according to the distance range for light reception of the light reflected by the object.
It is likely that blank portions are created locally in the extraction of an image in the distance range with the range image sensor, to lower the quality of the image. The occurrence of the blank portions is characteristically led by local portions in the blank portions with low reflectance to infrared rays. A signal level for the range image may be too low and hardly discernible from electric noise owing to insufficiency in the S/N ratio.
It is possible to compensate for the blank portions by use of pixels of portions in an image around the blank portions. However, compensation of all of the blank portions is not effective, because a certain object may intervene between the distance range and an image taking position to create one of the blank portions. No known technique can treat the blank portions in a suitable manner.